Implants have found applications in modern medical technology in manifold embodiments. For example, implants are used for supporting vessels, hollow organs, and duct systems (endovascular implants); for attaching and temporarily fixing tissue implants and tissue transplants; and for orthopedic purposes, for example, as nails, plates, or screws.
For radiological intraoperative and postoperative position monitoring, implants are provided with a marker if they do not already comprise a sufficiently radiopaque material. The x-ray visibility of the marker is a function of the dimensions and the x-ray absorption coefficient. The x-ray absorption coefficient is, in turn, a function of the energy range of the x-ray radiation. In the medical field, this is typically from 60 to 120 keV; for coronary use, a range of from 80 to 100 keV is typically employed. The x-ray absorption coefficient typically becomes larger with rising atomic number in the periodic table and the rising density of the material. The presence of the marker should not restrict the functionality of the implant or be a starting point for inflammation or rejection reactions of the body. Typically, for example, noble metals, such as gold, platinum and the like, are used as marker materials.
The markers are provided (i) as solid material, e.g., in the form of a coating, a strip, an inlay, or a molded body permanently bonded to the implant, or (ii) as powder embedded in a carrier matrix, in the form of a coating or as a filler material for a cavity in the implant. Variant (ii) may be implemented especially simply in production technology; a castable or sprayable mixture made of the radiopaque marker component and the material acting as a carrier matrix, possibly with a solvent added, is processed.
After fulfilling the therapeutic purpose, implants are removed operatively, for example, if this is still possible, because the implants remaining permanently in the body may result in inflammation or rejection reactions. An alternative to an operation is the use of biocorrodible materials for the implant. The number of biocorrodible materials based on polymers or metals is continuously growing. Thus, inter alia, biocorrodible metal alloys of the elements magnesium, iron, and tungsten are known. For example, European Patent Application No. 1 270 023 describes a magnesium alloy which is suitable for endovascular and orthopedic implants.
The biocorrodible metal alloys and polymers for medical implants known in the art have only slight x-ray visibility in the energy range of 80-100 keV, which is used for medical technology. However, x-ray diagnosis is an important instrument precisely for postoperative monitoring of the healing progress or for checking minimally invasive interventions. Thus, for example, stents have been placed in the coronary arteries during acute myocardial infarction treatment for some years. The stent is positioned in the area of the lesion of the coronary vascular wall and prevents obstruction of the vascular wall after expansion. The procedure of positioning and expanding the stent must be continuously monitored by the cardiologist during the procedure.
In implants made of biocorrodible metallic materials based on magnesium, iron, or tungsten, there are increased requirements for the marker material which include:                the marker is not to be detached early from the main body of the implant by the corrosive processes, to avoid fragmentation and thus the danger of embolization;        the marker is to have sufficient x-ray density even at low material thicknesses, and        the marker material is to have no or, at most, a slight influence on the degradation of the main body.        
German Patent Application No. 103 61 942 A1 describes a radiopaque marker for medical implants, which contains 10 to 90 weight-percent of a biocorrodible base component, in particular, from the group of elements consisting of magnesium, iron, and zinc. Furthermore, the marker contains 10 to 90 weight-percent of one or more radiopaque elements from the group consisting of I, Au, Ta, Y, Nb, Mo, Ru, Rh, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Bi, combinations thereof and the like as a marker component. The marker described is suitable in principle for use in biocorrodible implants, in particular, those made of biocorrodible magnesium alloys.
However, the special problem arises upon the use of markers made of metallic materials on biocorrodible metallic main bodies that, because of electrochemical interactions between the two metallic materials, the degradation of the main body is altered in a contact area between marker and main body, i.e., the degradation is typically accelerated.